I've got to admit to playing with myself for the last few days. Is my old sparring partner Alastair Campbell right about the Damian Green affair, or am I right? It's a tough one, isn't it, especially since more level-headed Labour types than Alastair are agreeing with his claim that the "tummy-tickling poodle press" have let the Tories, Green and the mayor of London, Boris Johnson, off the hook.

As you will certainly remember, Green, moderate Tory MP for Ashford and his party's immigration spokesman, was arrested on 27 November last year in connection with what we would once have called a special branch (now merged into counter-terrorism command) investigation into a string of Whitehall leaks traced to a junior civil servant called Christopher Galley.

Both men have now had their Met police bail extended from 17 February to 20 April. Whenever I bump into Green around the Westminster village I say: "How's it going?" and he expresses cheerful irritation at the sheer distraction of it all. "Now you know how Tony Blair felt," I remark and we go our separate ways.My initial reaction was that the arrest was likely to prove an error; excessive police tactics not sanctioned by Labour ministers, the MP "arrested for doing his job", as I wrote here, albeit with the elderly caveat that there just might be more to it than meets the eye, though there's usually less.

The fly in the woodpile is Mayor Boris. He was briefed in his new capacity as self-appointed chair of the Metropolitan police authority (MPA) and proceeded to both ring Green, an old friend and colleague, to discuss his plight, and then to express the very public view that the coppers had got it wrong.

Len Duvall, the ousted Labour chair of the MPA and a decent bloke, protested that the mayor had broken a whole string of rules. Jonathan Goolden a highly experienced local government solicitor and ex-public service manager, was appointed to examine Boris's behaviour.

That's what caused my weekend of introspection. As Dave Hill noted last month, Goolden concluded that Boris Johnson's conduct had been "extraordinary and unwise" in blabbing about an ongoing police investigation without consulting his press office and that a written protocol should be drawn up for future use.

But he was deemed not to have broken the rules, though the MPA and the Greater London authority standards board are still making their own assessment of the findings. Meanwhile, Duvall has issued his own stinging critique, explaining at some length why he believes Johnson did breach police confidentiality – as Goolden said he did not – in discussing the case with Green and publicising it.

Alastair Campbell's blog now describes how the media would have made a meal of the affair if a Labour politician such as Ken Livingstone had still been mayor – and accused him of helping out a political "crony" in a police investigation.

We sort of know this because the London Evening Standard, in the shape of the garrulous Andrew Gilligan, went after Livingstone in a protracted "Get Ken" campaign before last year's mayoral election.

The Labour MP and troublemaker Paul Flynn wittily called the process "innocent until proved Labour". And senior ministers I know sincerely assert that the cash-for-honours investigation would not have happened if the Tories had been in power.

It's true that most inquiries in the Thatcher-Major years, including the Guardian's battles with Jonathan Aitken and Neil Hamilton, were media-driven. In fairness to Len Duvall, he supported the Met's assistant commissioner, John Yates, throughout his investigation into the Blair peerages row; I know because I talked to him about it at the time. He's a straight arrow.

My problem with both cases, Blairgate and Greengate, is that both struck me as essentially political. The SNP used a Sunday Times report to shop Blair (and later won the Holyrood elections), though I never thought the police would make a viable case.

Nor does it currently look likely in the latest instance. So it seemed reasonable for politicians to regard it as such and act accordingly. Johnson thought Green's arrest outrageous and acted on that assumption. I still find it hard to disagree.

In any case, all sorts of people brief in ongoing criminal and civil cases, including the police themselves. You can spot their handiwork in the papers most days. They shouldn't, but they do and we – the media – happily accept the product. Our readers do too.

Wrong, but there it is. As I said at the time, I was – and remain – pretty confident that the cops were orchestrating the briefings on the cash-for-honours investigation, though they deny it.

There's another aspect of the way Green has been handled that Campbell seems reluctant to acknowledge. There's a cycle in politics, a tide that eventually turns against all incumbents. Failures stack up, voters and the media get bored, time for new faces.

Campbell took brilliant advantage of this cycle as John Major's government faded after 1992. Now the tide is turning against an 11-year-old Labour government, though, gut tribalist as he is, Campbell is back in the blogosphere fray – editing next week's New Statesman too – fighting to stave off defeat.

If it's any consolation (and in politics it rarely is) it means that Johnson will get away with his foibles, and failings will be tolerated for a while – good old Boris – but his time will come too. He will be stoned from office like everyone else. But not yet.